When you buy a cell phone in the United States, you normally sign up for a service plan with a wireless carrier, and are offered a phone to go with that plan, at a special price. What would happen if you were to buy a phone at its full retail price without a plan? Can you use it with any carrier you choose? Not without some effort. Here's why:

Wireless carriers in the U.S. lock their phones so that they can only be used with their service. This means that if you buy a T-mobile phone and put an AT&T SIM card in it, it won't work, giving users another disincentive beyond the cancelation agreement to stay with the original service. It is legal for carriers to lock their phones, but it is also legal for users to unlock them.

But why would anyone want to switch services? Well, besides being unhappy with a provider, if you travel outside the country for any period of time, you might want to use your same phone but get a local prepaid plan while you are in-country. So how can you unlock a phone? Here are some choices:

1: The best option: Buy a phone already unlocked. There are some stores, like Fry's, that are selling phones unlocked and ready for a variety of providers. You normally pay the full price of the phone, but you are not locked into any plan. You can also find phones for sale online as well, from retailers that buy phones overseas that have not been locked.You can buy used unlocked phones on Ebay as well. Make sure whomever you buy from has a return policy.

2. Call your carrier and ask them to unlock it. Every carrier has a different policy, but some carriers will unlock a phone for you if you have had a plan with them for a period of time. They will typically give you a code to enter into the phone and unlock it.

3. Go to a third-party provider that will unlock the phone. Some companies that repair cell phones can also unlock them, for a fee.

4. Buy an unlock code online. Sometimes you can find them for sale on Ebay or from companies that specialize in unlock codes. Be very careful about this one. They will ask you for the serial number of your phone and email you the unlock code and instructions. Make sure that you can get your money back if it doesn't work.

OK. So here are the requisite warnings:

• If you are going to unlock it yourself, be careful. Most phones will "freeze" after a number of unsuccessful attempts and have to be sent back to the manufacturer to be repaired. You won't get your money back for an unlock code if you freeze your phone.
• Check whether unlocking the phone will void the warranty. Unlocking an iphone, for example, is probably not worth it, because you will get no further support from Apple for software upgrades and the phone will be worthless if it gets frozen.
• If you are traveling overseas, check whether the phone you are unlocking or buying will work in that country.

Plenty of phones can be unlocked, or bought unlocked, however, and doing so will enable you to travel anywhere in the world where it is compatible and use whichever carrier you choose.